Transceiver filter and tuning

ABSTRACT

A radio transceiver including a transmitter and a receiver. A filter coupled to an output of the transmitter, the filter has one or more inductors and one or more capacitors, where the filter is tuned by varying one or more capacitance values of the one or more capacitors in the filter to tune the filter. The one or more capacitors are Barium Strontium Titanate (BST) ceramic integrated capacitors that are each tuned by application of a bias voltage thereto. A filter tuner circuit is configured to apply the bias voltage to the one or more capacitors, where the bias voltage is initially established as a stored initial value, and where the bias voltage is refined by a proportional-integral-derivative (PID) controller configured to optimize a power within the transceiver. This abstract is not to be considered limiting since various implementations may incorporate more, fewer or different elements.

PRIORITY CLAIM

This application claims priority to EP Application No. 12177875.7 filedon Jul. 25, 2012 entitled “Transceiver Filter and Tuning”, herebyincorporated herein in its entirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending U.S. application Ser. No.13/557,507 filed on Jul. 25, 2012 entitled “Transceiver Filter andTuning”, hereby incorporated herein in its entirety.

BACKGROUND

In modern radios such as those used in cellular telephones, a highperformance receiver path is used to detect signals below −110 dBm inthe presence of blocking signals up to −20 dBm. This makes design of theradio and the frequency synthesizer challenging in terms of reducingpower consumption. The receiver (Rx) path may also be used to sense thetransmitter (Tx) signal for either envelope tracking or tuning theantenna. Furthermore, the receiver may be used to detect blockers orjamming signals to improve the quality of the wanted signal. Thisintroduces practical implementation problems such as VCO (voltagecontrol oscillator) pulling in the receiver.

In radio designs using full duplex (simultaneous Rx and Tx) modes ofoperation, such as those used in certain code division multiple access(CDMA) cellular telephone and next generation technologies, transmittedsignals entering the receiver input can be an especially significantproblem. In these technologies, an auxiliary Rx channel is sometimesused for power level sensing and balancing of both receiver andtransmitter. These additional channels are known, and commercial ICs forimplementing such auxiliary channels exist.

Consider the design of a conventional full duplex radio transceiver suchas that depicted in FIG. 1. In such a design, a transmitter 100 outputsignal is passed to an antenna 104 through a switch 108. Since modernreceiver designs may operate over multiple bands, multiple transmittersand receivers may be represented by transmitter 100 and receiver 112.Since the transmitter and receiver share the same antenna, for each bandof frequencies used by the receiver, the design shown in FIG. 1 utilizesa separate filter 116, 120 through 124 (e.g., a SAW filter or the like)configured as notch filters. Switch 108 switches to the correct notchfilter for the band of transmitted signals based on commands from acontrol processor 130 based on a selected channel or band. These notchfilters are used to prevent the relatively high power from the RFamplifier 134 of transmitter 100 from entering the front end low noiseamplifier of the receiver 112 and either damaging the receiver ordegrading operation thereof.

As worldwide radio receivers are developed, as many as 20 (or possiblymore) bands of frequencies may need to be accommodated to truly handleeach possible frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present disclosure will be described belowwith reference to the included drawings such that like referencenumerals refer to like elements and in which:

FIG. 1 is block diagram of an example full duplex radio design formultiple frequency bands.

FIG. 2 is an exemplary implementation of a variable filter arrangementconsistent with certain embodiments of the present invention.

FIG. 3 is an exemplary implementation of a filter tuner circuitconsistent with certain embodiments of the present invention.

FIG. 4 is an example of a process flow for a filter tuning processconsistent with certain embodiments of the present invention.

DETAILED DESCRIPTION

The various examples presented herein outline methods, user interfaces,and electronic devices that allow a multiple band radio to operatewithout use of large numbers of individual filters for each of aplurality of radio bands.

For simplicity and clarity of illustration, reference numerals may berepeated among the figures to indicate corresponding or analogouselements. Numerous details are set forth to provide an understanding ofthe embodiments described herein. The embodiments may be practicedwithout these details. In other instances, well-known methods,procedures, and components have not been described in detail to avoidobscuring the embodiments described. The description is not to beconsidered as limited to the scope of the embodiments described herein.

The terms “a” or “an”, as used herein, are defined as one or more thanone. The term “plurality”, as used herein, is defined as two or morethan two. The term “another”, as used herein, is defined as at least asecond or more. The terms “including” and/or “having”, as used herein,are defined as comprising (i.e., open language). The term “coupled”, asused herein, is defined as connected, although not necessarily directly,and not necessarily mechanically. The term “program” or “computerprogram” or similar terms, as used herein, is defined as a sequence ofinstructions designed for execution on a computer system. A “program”,or “computer program”, may include a subroutine, a function, aprocedure, an application or “app”, an object method, an objectimplementation, in an executable application, an applet, a servlet, asource code, an object code, a script, a program module, a sharedlibrary/dynamic load library and/or other sequence of instructionsdesigned for execution on a computer system.

Therefore, in accordance with certain aspects of the present disclosure,there is provided a radio transceiver having a transmitter and areceiver. A filter is coupled to an output of the transmitter, thefilter having one or more integrated capacitors, where the filter istuned by varying one or more capacitance values of the one or moreintegrated capacitors in the filter to tune the filter, at least one ofthe one or more integrated capacitors comprise Barium Strontium Titanate(BST) ceramic integrated capacitors that are each tuneable byapplication of a bias voltage thereto. A filter tuner circuit isconfigured to apply the bias voltage to the one or more integratedcapacitors, where the bias voltage is initially established as a storedinitial value. A proportional-integral-derivative (PID) controller isconfigured to refine the bias voltage to optimize a power within thetransceiver.

In certain implementations, the filter is a notch filter coupled betweena transmitter output and a receiver input of the transceiver that blockstransmitter power from entering the receiver. In certainimplementations, the PID controller is configured to maximize adifference between receiver frequency power and transmitter frequencypower at the receiver input. In certain implementations, the filtertuner circuit has a low pass filter configured to receive transmittersignals present at the receiver input, a power calculator configured tocalculate power received from the low pass filter, and an errorcalculator configured to calculate a difference between the calculatedpower and receive frequency power present at the receiver input, wherethe PID controller is configured to maximize a difference between thereceiver frequency power and transmitter frequency power at the receiverinput. In certain implementations, the PID controller is configured tomaximize a difference between receiver frequency power and transmitterfrequency power at the receiver input. In certain implementations, thefilter is a notch filter.

In certain implementations, the transmitter and receiver described aboverepresent a plurality of transmitters and receivers operable over threebands, where the three bands comprise a low band between 700 Mhz and 1Ghz, a middle band between 1.8 Ghz and 2.2 Ghz and a high band between2.3 Ghz and 2.7 Ghz. In certain implementations, the filter describedabove represents three filters, one for each of the three bands, andwhere each of the three filters are coupled directly to one of threereceivers. In certain implementations, the one or more integratedcapacitors are exposed to transmitter power in excess of 20 dBm.

A radio transceiver consistent with certain implementations has amulti-band transmitter and a multi-band receiver, where the transmitterand receiver are configured to operate in full duplex with a transmitterchannel for use by the transmitter and a receiver channel for use by thereceiver. An antenna is shared by the transmitter and the receiver forfull duplex communication, where the transmitter has an output coupledto the receiver and the receiver has an input coupled to the antenna. Anotch filter made up of one or more integrated capacitors is disposedbetween the antenna and the receiver. The notch filter is configured tobe tuned by varying one or more capacitance values of the one or moreintegrated capacitors in the notch filter to select the transmitterchannel for rejection by the notch filter, at least one of the one ormore integrated capacitors comprising Barium Strontium Titanate (BST)ceramic integrated capacitors that are each tuneable by application of abias voltage thereto. A filter tuner circuit is configured to apply thebias voltage to the one or more integrated capacitors, where the biasvoltage is initially established as a stored initial value, and wherethe bias voltage is refined by a proportional-integral-derivative (PID)controller configured to optimize a power in the transceiver.

In certain implementations, the PID controller is configured to maximizea difference between receiver frequency power and transmitter frequencypower at the receiver input. In certain implementations, the filtertuner circuit is a low pass filter configured to receive transmittersignals present at the receiver input with a power calculator configuredto calculate power received from the low pass filter, and an errorcalculator configured to calculate a difference between the calculatedpower and receive frequency power present at the receiver input, wherethe PID controller is configured to maximize a difference betweenreceiver frequency power and transmitter frequency power at the receiverinput.

In certain implementations, the multi-band transmitter and multi-bandreceiver described above represents a plurality of transmitters andreceivers operable over three bands, where the three bands comprise alow band between 700 Mhz and 1 Ghz, a middle band between 1.8 Ghz and2.2 Ghz and a high band between 2.3 Ghz and 2.7 Ghz. Similarly, thefilter described above represents three filters, one for each of thethree bands, and where each of the three filters are coupled directly toone of three receivers. In certain implementations, the one or moreintegrated capacitors are exposed to transmitter power in excess of 20dBm.

An example method of tuning a filter in a radio transceiver, involvesproviding a tuneable filter forming a part of the transceiver having anintegrated capacitor, where the integrated capacitor comprises as BariumStrontium Titanate (BST) ceramic integrated capacitor that is tuneableby application of a bias voltage thereto, where the transceiver has atransmitter and a receiver; retrieving a stored initial value of thebias voltage from a memory; and refining the initial value of the biasvoltage by using a proportional-integral-derivative (PID) controller toproduce a refined bias voltage and applying the refined value biasvoltage to the capacitor to optimize a signal in the radio transceiver.

In certain implementations, the method further involves applying theinitial value of the bias voltage to the capacitor prior to refining theinitial value. In certain implementations, the refining comprisesmaximizing a difference between a power transmitted by the transmitterand a power from the transmitter appearing at the input of the receiver.In certain implementations, the filter comprises a notch filter coupledbetween a transmitter output and a receiver input of the transceiverthat blocks transmitter power from entering the receiver. In certainimplementations, the refining comprises maximizing a difference betweenreceiver frequency power and transmitter frequency power at the receiverinput. In certain implementations, the transmitter and receiver comprisea plurality of transmitters and receivers operable over three bands,where the three bands comprise a low band between 700 Mhz and 1 Ghz, amiddle band between 1.8 Ghz and 2.2 Ghz and a high band between 2.3 Ghzand 2.7 Ghz. In certain implementations, the filter discussed aboverepresents three filters, one for each of the three bands, and whereeach of the three filters are coupled directly to one of threereceivers. In certain implementations, the integrated capacitor isexposed to transmitter power in excess of 20 dBm.

As noted above, as many as 20 bands of channels are to be accommodatedin order to provide a single radio that operates worldwide.Unfortunately, the mechanism used in FIG. 1 for isolating the sensitiveinput of the receiver from high powers from the transmitter would resultin use of a complex switch circuit 108 and perhaps twenty or more notchfilters 116, 120 through 124. Such filters while having a relativelysmall footprint individually, will occupy a considerable amount of spacewhen twenty such filters are used. Moreover, the cost of twenty filtersincreases the cost of the radio and constrains the how small the radiocan be.

An improvement can be made by utilizing tuneable capacitors to implementtraditional inductor and capacitor filter structures. But simplysubstituting LC filter designs that utilize conventional variableintegrated capacitors may be detrimental to the life of the radio andmay not conserve valuable real estate. Conventionalmetal-insulator-metal (MIM) and metal-oxide-metal (MOM) integratedcapacitors may fail or degrade if exposed to high voltage and power. Forexample, such capacitors are generally limited to voltages of 3.0 voltsmaximum to achieve long term reliable performance. This corresponds to amaximum power of about 13-14 dBm. But, 3G and 4G LTE cellular radiotransmitters can output between about 8 and 20 volts to provide outputpower in excess of 20 dBm and generally between about 22 and 33 dBm. Thefilters may be exposed to such powers in use for up to 5-10 years. Usingconventional MIM and MOM integrated capacitors to implement such filterswill surely result in premature failure or performance degradation, andlikely catastrophic failure of the radio.

In order to achieve a reasonable substitution of variable LC filters inan integrated radio system, conventional capacitors such as MIMcapacitors or MOM capacitors are clearly unsuitable. However, it hasbeen found that doped Barium Strontium Titanate (BST) integrated ceramiccapacitors such as those manufactured by Paratek Microwave, Inc. canreliably handle power in the range of 40 dBm reliably for long periodsof time. Additionally, in the present application, twenty bands offilters can be handled with only three variable filters for high, middleand low band frequency ranges. For current frequency allocations, thiscan be done with a low band between 700 Mhz and 1.0 Ghz, a middle bandof 1.86 Ghz to 2.2 Ghz and a high band between 2.3 Ghz and 2.7 Ghz, witheach band handling six to seven channels.

In the present case, this means that only three variable integratedvariable capacitor based filters can be used to replace twenty fixedfilters. Since suitable BST based filters designs occupy approximatelythe same footprint as a single fixed SAW filter, the size savings withinthe radio are substantial (3/20=0.15 for approximately an 85% savings).Additionally, since a separate receiver is used for each of the threebands (high, low and middle) the switch can be eliminated by simplyattaching the variable filter to the front end of each of the threereceivers. Thus, each variable filter is paired with a radio receiver ofthe same band. Any suitable notch filter design, for the presentimplementation, can be utilized.

An example receiver has the following elements: The receiver is coupledto the output of the transmitter (i.e. near the antenna) via a coupler.This then goes through a tuneable LC (inductive-capacitive) tank thatuses BST tuneable integrated capacitors. This is followed by thereceiver pre-amp or attenuator, and then a mixer that is attached to avoltage controlled oscillator (VCO) and phase locked loop (PLL). The VCOuses an oscillator that has high frequency protection on the suppliesand grounds. After the mixer, filtering and automatic gain control (AGC)follow and then a high band width low current analog to digitalconverter (ADC) (for example a 100 MHz SAR ADC). The output of ADC arethen applied to the various digital signal processing (DSP) blocks foreither envelope tracking, antenna tuning, or detecting the contents ofthe RX spectrum.

FIG. 2 is a simplified illustration of an example electronic device 200in accordance with aspects of the present disclosure. In this exampleradio, the bank of filters depicted in FIG. 1 is replaced with one ormore variable filters 204. These variable notch filters 204 are coupleddirectly to the input RF amplifier (i.e., the “front end”) 208 of thereceivers 210 in accord with certain implementations (only one bandshown for simplicity and ease of illustration, but it will be understoodthat multiple filters and receivers are utilized to cover all twentybands). The variable LC filter 204 is configured as any desired LC notchfilter circuit in this application so that power from the transmitter206's output power amplifier 212 which is intended for antenna 216 islargely blocked from the input of receiver RF amplifier 208. The RFamplifier 208 is coupled to the remainder of the receiver which is shownin part by mixer 220 (which mixes the incoming receiver channel signalwith the receiver frequency F_(RF) for direct conversion to baseband)and ADC 224 for operation in a more or less conventional manner whichneed not be discussed in detail for purposes of this discussion.

The signal at the receiver input is also utilized to control the tuningof the variable integrated capacitor or capacitors used in the LC filter204 by amplifying the signal using RF amplifier 230 to condition thesignal to a suitable level for mixing at mixer 234 with the receiverfrequency using a local oscillator at the transmitter frequency F_(TX).Hence, signals at the receive frequency and the transmit frequency areavailable for measurement and calculation of adjustments for the filter204 using the filter tuner 250.

In order to implement a suitable filter controller 250, the filtercontroller is designed and configured so that the adjusted frequency ofthe filter is manipulated such that the transmit signal is effectivelynotched out. In this example implementation, aproportional-integral-derivative (PID) controller is chosen to do this.Since the carrier frequency of the transmitter is known by virtue of thechannel selection mechanism of the radio, this information can be usedto initially coarsely tune the capacitor(s) of the LC filter(s) 204. ThePID controller can then be used in a feedback loop as shown with thefilter tuner 250 feeding back control signals to LC filter 204 to adjustthe capacitor values to achieve an optimized tuning. Any suitableoptimization technique can be utilized by the PID controller withoutlimitation.

FIG. 3 is a block diagram of an example functional representation of thefilter tuner 250. In accord with this example filter tuner 250, thebaseband transmit signal is low pass filtered to eliminate mixingartefacts at filter 304. This filtered baseband transmit signal ispassed to a power calculator block 308 that calculates the power in theresidual transmit signal present at the input of the receiver. Thiscalculated power is then sent to error calculator 312 that compares thispower to a reference value and the output is provided to a PIDcontroller 318 for processing. The initial value or values used toinitially set the filter's tuning is stored in memory 324 and that valueis initially loaded into the LC filter 204 when the channel is selected.The PID controller modifies this value iteratively to minimize theamount of power from the transmitter that is received at the receiver.In one example implementation, this can be effectively accomplished bymaximizing the difference between receiver frequency power P_(RX) andtransmit frequency power P_(TX) at the receiver input (i.e.,MAX(P_(RX)−P_(TX))), where in this example, the reference value providedto the error calculator is the receive frequency power. Thismaximization process accounts for any effects the notch filter has onthe receive frequency power as well as the transmit frequency power.

In the present example implementation, any number of filter tunerimplementations can be utilized. FIG. 4 depicts one exampleimplementation process 400 starting at 402. When a channel is tuned at406, by any suitable mechanism, initial filter values may be loaded intothe tuneable LC filter 204 at 410. These initial values may be factorygenerated as approximate values for a given design, or may be measuredand stored for a particular production radio. Even if the initial valueis selected for an individual production radio, the RF filter based oninductors and capacitors values will desirably be tuned to optimize thefilter characteristics since given components will drift in values withage, changes in temperature, etc. Once the filter has been adjustedaccording to the present process, the value used most recently may bestored as a new initial value on a channel by channel basis or theoriginal default may be used each time without limitation.

The transmit frequency signal at the receiver input is low pass filteredto remove the unneeded mixing components at 414 and the transmit powerat the receiver input is calculated from the signal at 418. The errorfrom the reference values (the receiver power signal) is calculated at422 and a correction factor is computed at 426 using the PID controllerin order to optimize the power difference between the receiver andtransmit power at 430. The calculation of a correction factor isiterated in the loop made up of 418, 422, 426 and 430 until the filtervalue is optimized at 430. Once this correction factor is optimized itmay be monitored periodically or simply left in place at 430. Thisprocess repeats itself starting at 410 whenever a new channel isselected at 440.

Those skilled in the art will appreciate that this process may beinterrupted during the optimizing process should a channel change beeffected prior to full optimization, but the basic process can beunderstood by reference to the process 400 for illustrative purposes.Other variations will occur to those skilled in the art uponconsideration of the present teachings. For example, the optimizationcan be carried out by minimizing the transmit power at the receiverinput or by other techniques than those depicted herein withoutdeparting from embodiments consistent with the present invention.

The order in which the optional operations represented in the flow chart400 may occur in any operative order without limitation. Thus, while theblocks comprising the methods are shown as occurring in a particularorder, it will be appreciated by those skilled in the art that certainof the blocks may be rearranged and can occur in different orders and beaugmented by other process functions than those shown without materiallyaffecting the end results of the methods.

The implementations of the present disclosure described above areintended to be examples only. Those of skill in the art can effectalterations, modifications and variations to the particular exampleembodiments herein without departing from the intended scope of thepresent disclosure. Moreover, selected features from one or more of theabove-described example embodiments can be combined to createalternative example embodiments not explicitly described herein. Forexample, while the embodiment discussed above utilizes a notch filterthat prevents excessive transmitter power from entering the receiver,other example implementations could use a band pass, low pass or highpass filter configuration where tuning of the filter is accomplished byoptimization of a power or power difference (both referred to herein asa power).

It will be appreciated that any module or component disclosed hereinthat executes instructions may include or otherwise have access tonon-transitory and tangible computer readable media such as storagemedia, computer storage media, or data storage devices (removable ornon-removable) such as, for example, magnetic disks, optical disks, ortape data storage. In this document, the term “non-transitory” is onlyintended to exclude propagating waves and signals and does not excludevolatile memory or memory that can be rewritten or erased. Computerstorage media may include volatile and non-volatile, removable andnon-removable media implemented in any method or technology for storageof information, such as computer readable instructions, data structures,program modules, or other data. Examples of computer storage mediainclude random access memory (RAM), read only memory ROM, electricallyerasable programmable read only memory (EEPROM), flash memory or othermemory technology, compact disc ROM (CD-ROM), digital versatile disks(DVD) or other optical storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium which can be used to store the desired information and which canbe accessed by an application, module, or both. Any such computerstorage media may be part of the server, any component of or related tothe network, backend, etc., or accessible or connectable thereto. Anyapplication or module herein described may be implemented using computerreadable/executable instructions that may be stored or otherwise held bysuch computer readable media.

The present disclosure may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the disclosure is, therefore,indicated by the appended claims rather than by the foregoingdescription. All changes that come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. A radio transceiver, comprising: a transmitterand a receiver; a transmit filter coupled to an output of thetransmitter, the transmit filter comprised of one or more integratedcapacitors, where the transmit filter is tuned by varying one or morecapacitance values of the one or more integrated capacitors in thetransmit filter to tune the transmit filter, at least one of the one ormore integrated capacitors comprise Barium Strontium Titanate (BST)ceramic integrated capacitors that are each tuneable by application of abias voltage thereto; a filter tuner circuit comprising a receive filterconfigured to receive transmitter signals present at a receiver input; apower calculator configured to calculate power received from the receivefilter; an error calculator configured to calculate a difference betweenthe calculated power and receive frequency power present at the receiverinput, wherein the filter tuner circuit is configured to apply the biasvoltage to the one or more integrated capacitors, where the bias voltageis initially established as a stored initial value; and aproportional-integral-derivative (PID) controller, where the PIDcontroller is configured to refine the bias voltage to maximize adifference between the receiver frequency power and transmitterfrequency power at the receiver input.
 2. The radio transceiveraccording to claim 1, where the transmit filter is a notch filtercoupled between the transmitter output and the receiver input of theradio transceiver that blocks transmitter power from entering thereceiver.
 3. The radio transceiver according to claim 1, where thetransmit filter is a notch filter.
 4. The radio transceiver according toclaim 1, where the transmitter and receiver comprise a plurality oftransmitters and receivers operable over three bands, where the threebands comprise a low band between 700 Mhz and 1 Ghz, a middle bandbetween 1.8 Ghz and 2.2 Ghz and a high band between 2.3 Ghz and 2.7 Ghz.5. The radio transceiver according to claim 4, where the transmit filtercomprises three filters, one for each of the three bands, and where eachof the three filters is coupled directly to one of three receivers. 6.The radio transceiver according to claim 1, where the one or moreintegrated capacitors are exposed to transmitter power in excess of 20dBm.
 7. The radio transceiver according to claim 1, where thetransmitter and receiver comprise a multi-band transmitter and amulti-band receiver, where the transmitter and receiver are configuredto operate in full duplex with a transmitter channel for use by thetransmitter and a receiver channel for use by the receiver, the radiotransceiver further comprising: an antenna shared by the transmitter andthe receiver for full duplex communication, where the output of thetransmitter is coupled to the receiver and the input of the receiver iscoupled to the antenna; where the transmit filter comprises a notchfilter.
 8. The radio transceiver according to claim 7, where themulti-band transmitter and multi-band receiver are operable over threebands, where the three bands comprise a low band between 700 Mhz and 1Ghz, a middle band between 1.8 Ghz and 2.2 Ghz and a high band between2.3 Ghz and 2.7 Ghz.
 9. The radio transceiver according to claim 8,where the transmit filter comprises three filters, one for each of thethree bands, and where each of the three filters is coupled directly toone of three receivers.
 10. The radio transceiver according to claim 7,where the one or more integrated capacitors are exposed to transmitterpower in excess of 20 dBm.
 11. The radio transceiver according to claim1, where the receive filter comprises a low pass filter.
 12. A method oftuning a filter in a radio transceiver, comprising: providing a tuneablefilter forming a part of the radio transceiver and comprising anintegrated capacitor, where the integrated capacitor comprises a BariumStrontium Titanate (BST) ceramic integrated capacitor that is tuneableby application of a bias voltage thereto, where the radio transceiverhas a transmitter and a receiver; providing a receive filter configuredto receive transmitter signals present at a receiver input; providing apower calculator configured to calculate power received from the receivefilter; providing an error calculator configured to calculate adifference between the calculated power and receive frequency powerpresent at the receiver input, retrieving a stored initial value of thebias voltage from a memory; and refining the initial value of the biasvoltage by using a proportional-integral-derivative (PID) controller toproduce a refined bias voltage to maximize a difference between thereceiver frequency power and transmitter frequency power at the receiverinput; and applying the refined bias voltage to the integratedcapacitor.
 13. The method according to claim 12, further comprisingapplying the initial value of the bias voltage to the integratedcapacitor prior to refining the initial value.
 14. The method accordingto claim 12, where the tuneable filter is a notch filter coupled betweena transmitter output and the receiver input of the radio transceiverthat blocks transmitter power from entering the receiver.
 15. The methodaccording to claim 12, where the transmitter and receiver comprise aplurality of transmitters and receivers operable over three bands, wherethe three bands comprise a low band between 700 Mhz and 1 Ghz, a middleband between 1.8 Ghz and 2.2 Ghz and a high band between 2.3 Ghz and 2.7Ghz.
 16. The method according to claim 15, where the tuneable filtercomprises three filters, one for each of the three bands, and where eachof the three filters is coupled directly to one of three receivers. 17.The method according to claim 12, where the integrated capacitor isexposed to transmitter power in excess of 20 dBm.